Supercritical carbon dioxide drying

Mulhern, G.T. Soane, D.S. Howe, R.T. Supercritical carbon dioxide drying of microstructures. In Technical Digest, 7th International Conference on Solid-State Sensors and Actuators, Jun 1993 296-299. 

Controlled increase in the surface area of isolated chitins and chitosans certainly results in more convenient chemical and biochemical reactivity. Likewise, controlled porosity is a means for optimizing the growth of human or animal cells within scaffolds. The mechanical disassembly of animal chitins under controlled conditions represents an important step forward in the exploitation of nanochitins and nanochitosans. Along with supercritical carbon dioxide drying, mechanical disassembly exhibits practical advantages over electrospinning. 

Tang Q, Wang T (2005) Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying. 

Figure 9.9. Photographs of cellulose gels according to reference [28] A - hydrogel in water B - alcogel in EtOH and C - aerogel obtained by supercritical carbon dioxide drying (with permission of Wiley-VCH Verlag GmbH Co).

Mastrangelo CH, Hsu CH (1993) Mechanical stability and adhesion of microstructures under capillary forces - part I basic theory. J Microelectromech Syst 2(l) 33-43 Mulhem GT, Soane DS, Howe RT (1993) Supercritical carbon dioxide drying of microstructures. In Technical digest 7th international conference on solid-state sensors and actuators (Transducers 93), Yokohama, p 296... 

Hu, Z.S., Dong, J.X., Chen, G.X., 1999, Preparation of nanometer copper borate with supercritical carbon dioxide drying. Powder Technol. 102, 171-176. 

G.T. Mulhem, D.S. Soane, and R.T. Howe, Supercritical carbon dioxide drying of microstructures, Proc. Transducers 93 (Yokohama), pp. 296-299 (1993). 

It was proposed that supercritical carbon dioxide extraction inhibits mesopore collapse leaving micropores accessible to gases. It is apparent that supercritical carbon dioxide drying has advantages in activating the porous structures of MOFs. 

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Another challenge is to develop methods to replace the volatile organic solvents that are used in many industrial procedures. One choice is water as a solvent it is easily repurified, and has a harmless vapor. Another choice is supercritical carbon dioxide, a good solvent for many organic substances. It is not as innocuous as is water, but carbon dioxide can be easily recovered and reused. It is currently used to remove caffeine from coffee, and is being developed as a dry-cleaning solvent to replace organic solvents (Chapter 9). 

A 1-g sieved, air-dry soil sample is placed in an extraction cell with methanol as a modifier. The sample is extracted at a C02 flow rate of 1.5 mT/min with supercritical carbon dioxide for 15 minutes and analytes trapped in an octadecyl siloxane microextraction disk for subsequent analysis (adapted and condensed from Reference 9). 

Carbon dioxide, as can most other substances, can exist in any one of three phases—solid, liquid, or gas—depending on temperature and pressure. At low temperatures, carbon dioxide exists as a solid ("dry ice") at almost any pressure. At temperatures greater than about -76°F (-60°C), however, carbon dioxide may exist as a gas or as a liquid, depending on the pressure. At some combination of temperature and pressure, however, carbon dioxide (and other substances) enters a fourth phase, known as the supercritical phase, whose properties are a combination of gas and liquid properties. For example, supercritical carbon dioxide (often represented as SCC02, SC-C02, SC-CO2, or a similar acronym) diffuses readily and has a low viscosity, properties associated with gases, but is also a good solvent, a property one often associates with liquids. The critical temperature and pressure at which carbon dioxide becomes a supercritical fluid are 31.1°C (88.0°F) and 73.8 atm (1,070 pounds per square inch). 

Examines the supercritical extraction of dry tobacco powder using supercritical carbon dioxide (SC-CO2) as solvent (Rincon et al., 1998). 

Table I. Supercritical Carbon Dioxide Efficienqr for Removing Fat from Dried Mnscie Foods...

Bacterial mutagenesis tests have been conducted with distilled water solutions of the freeze-dried residues [concentrated up to 3000-fold (7)] and partially freeze-dried samples [concentrated 10-fold (49)]. High salt concentrations in such concentrates may cause toxicity problems in the bacterial tests. The use of dimethyl sulfoxide, methanol, or supercritical carbon dioxide to extract the organics from the freeze-dried residues for mutagenicity test purposes should be investigated. 

The supercritical carbon dioxide extract stream is passed from the extractor vessel outlet through pressure reduction valve 8, where the pressure is reduced to atmospheric pressure and the extracted organic substance is precipitated in collection device 9. The atmospheric pressure carbon dioxide then flows from the collection device through a rotameter 10 and dry gas meter 11, which measure CO2 flow rate and total volume, respectively, to the vent 12. 

The results obtained clearly demonstrate that sulfate ions promote the consolidation of titania morphology in nanometer scales and the formation of a crystalline, anatase phase in aerogels dried using supercritical carbon dioxide. This trend is consistently demonstrated by adsorption experiments as well as SAXS and XRD studies. The presence of platinum promotes the formation of a fine polymeric structure of titania in nanometric scales. After calcination all samples exhibit a similar morphology, yet with a notable difference in texture parameters. 

In contrast to the decaffeination of coffee, which is primarily executed with green coffee, black tea has to be extracted from the fermented aromatic material. Vitzthum and Hubert have described a procedure for the production of caffeine-free tea in the German patent application, 2127642 [11]. The decaffeination runs in multi-stages. First, the tea will be clarified of aroma by dried supercritical carbon dioxide at 250 bar and 50°C. After decaffeination with wet CO2 the moist leaf-material will be dried in vacuum at 50°C and finally re-aromatized with the aroma extract, removed in the first step. Therefore, the aroma-loaded supercritical CO2 of 300 bar and 40°C will be expanded into the extractor filled with decaffeinated tea. The procedure also suits the production of caffeine-free instant tea, in which the freeze-dried watery extract of decaffeinated tea will be impregnated with the aromas extracted before. 

Extraction of fat by supercritical carbon dioxide was investigated as an important option for minimizing the expanded use of frequently flammable and carcinogenic solvents in food analysis. Unfortunately, the presence of moisture in foods has an adverse effect on the quantitative extraction of fat by supercritical fluid extraction (SEE). Hence, samples have to be lyophilized first. The total fat content of freeze-dried meat and oilseed samples was found to be comparable to values derived from Soxhlet-extracted samples (26). Besides, only small amounts of residual lipids could be recovered by an additional extraction of the SFE-extracted matrix by the Bligh and Dyer solvent extraction procedure. As far as the minor constituents are concerned, it was found that the extraction recovery ranged from 99% for PC to 88% for PA. Hence, Snyder et al. concluded that SFE can be used as a rapid, automated method to obtain total fat, including total phospholipids, from foods (26). 

The flavour and modern phytopharmaceutical industries have made big changes to the traditional pharmaceutical extraction processes. Whereas ethanol was really the only significant solvent apart from water used by the traditional pharmaceutical extractors, solvents such as hexane and acetone have been used by flavour companies to make soft-extract oleoresins for natural flavour components. Sub- and supercritical carbon dioxide and also some fluorohydrocarbons are now used to produce some very high-quality extracts. Modern concentration and drying processes such as reverse osmosis, spray-drying and freeze-drying... 

Drying method drying agent temperature pressure time CO2 flow rate Semicontinuous supercritical carbon dioxide (CO2) 343 K 2.1 x 107 Pa 2h 85 Lh downstream Semicontinuous supercritical carbon dioxide (C02) 343 K 2.1 x 107 Pa 2h 85 Lh-1 downstream... 

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